High blood pressure is a potent risk factor for cardiovascular, cerebrovascular, and kidney disease. Although it is highly heritable, the genetic causes of blood pressure variation in the general population have been ill-defined. An important challenge to the study of blood pressure is its multifactorial origins, motivating the division of hypertension into discrete physiologic subtypes. Abnormal salt-handling has received much attention, given the implications of salt-sensitivity for behavioral and pharmacologic interventions. Several physiologic systems have evolved to handle salt, and these may contribute to blood pressure regulation. The best- studied system is the renin-angiotensin-aldosterone system (RAAS), which likely evolved to promote salt retention in hot climates with limited access to dietary salt. The endogenous system that counter-balances the RAAS is the natriuretic peptide (NP) system. The NP are natriuretic and vasodilatory molecules produced by the heart in response to increased wall stress in the atria and ventricles. Relatively little is known regarding the homeostatic role of NP in healthy individuals. The low resting levels of NP in ambulatory individuals and the involvement of the NP axis in a feedback loop have presented challenges to defining the physiologic implications of alterations in NP activity. In preliminary work, we have identified a common genetic variant at the NPPA/NPPB locus associated with resting NP concentrations and blood pressure. Using data from up to 60,000 individuals, we observed that genetically-determined lower NP levels were related to higher blood pressure and increased risk of hypertension, directly implicating the NP in human blood pressure regulation. In Aim 1, we seek to fine map the NPPA/NPPB common variant association and identify novel low-frequency, high-effect alleles through large-scale resequencing of the locus. In Aim 2, we will assess the impact of genetically- determined low NP levels on salt-handling by recruiting individuals with genetically low and with genetically high NP levels and assessing the response to intravenous saline challenge on the background of low- and high-salt diets. In Aim 3, we will manipulate the human NPPA/NPPB locus using a bacterial artificial chromosome in cellular systems to establish the variants that are sufficient to alter NPPA or NPPB expression. The investigators' proposed use of high- throughput resequencing and genotyping, detailed physiologic phenotyping, and molecular characterization of the DNA sequence variation identified promise to yield fundamental insights into structure/function relationships at the NPPA/NPPB locus, as well as elucidate the role of NP in acute and chronic salt responses and blood pressure regulation in the general population.